Many synthetic rubbers or elastomers are produced by emulsion polymerization. This process forms a latex directly, the polymer being the dispersed phase and water being the continuous phase. Nitrile rubber and styrene-butadiene rubber are prominent examples of synthetic rubbers that can be prepared in this manner. Other synthetic rubbers cannot be formed by emulsion polymerization, and instead require solution polymerization in a hydrocarbon solvent. Prominent among these are polybutadiene rubbers and polyisoprene rubbers, notably cis-1,4-polyisoprene. To produce a latex from these polymers, the polymer must be emulsified as a hydrocarbon solution with water and an emulsifying agent followed by removal of the solvent from the emulsion. Synthetic polyisoprenes vary in their content of cis-1,4-polyisoprene, those with a higher content having properties closer to those of natural rubber.
Solvent removal is a difficult step in the manufacture of synthetic rubber. When the emulsion is heated to vaporize the solvent, foam or froth forms, which interferes with the formation of the latex, particularly by prolonging the processing time. Foam formation can be reduced by using a reduced heating rate, but this is economically unfavorable since it prolongs the processing time. The use of low-foaming surfactants to reduce foaming is disclosed by Preiss et al., U.S. Pat. No. 3,215,649, but these merely reduce the foaming rather than eliminate it. The use of flash distillation followed by tangentially intersecting the resulting foam with a heated vapor stream is disclosed by van Hardeveld et al., U.S. Pat. No. 3,886,109. This requires special surfactants and is a costly process which produces a latex of questionable stability.
In general, the rate of solvent removal by distillation, gas stripping or steam stripping is limited by the rate at which foam can be dissipated or actively broken by applied shear forces. If heat is applied too rapidly, the emulsion may overheat and foam over. Stripping systems of very large volume will be required to accommodate the large amount of slowly breaking foam. Even foams with low-foaming anionic surfactants require a significant amount of time to break. Steam stripping frequently impairs the stability of the resulting latex, causing coagulation. While the tendency toward coagulation can be reduced by chemical modification of the polymer, the modified polymer is thus less similar in structure to natural latex rubber and consequently has less favorable properties. The reductions furthermore do not eliminate the coagulation entirely.
The use of chemical defoaming agents has also been disclosed. Many of these agents are detrimental to the properties of the final latex, however, causing for example pin-hole formation. Mechanical means of breaking foams such as stirrers, baffles and air jets can lower the stability of the foam which will result in a greater tendency toward coagulation. Foams containing some surfactants, such as sodium lauryl sulfate, are not capable of being broken by mechanical means, even with extremely high shear forces. The high shear force will merely reduce the bubble size in these foams, rendering the foam even more difficult to control.
The use of rotary evaporators for foam control has been disclosed, but the centrifugal force in these evaporators can destabilize the emulsion or impose a compressive force on the liquid, making evaporation of the solvent more difficult.